The present invention relates to a catalyst for transesterification.
As a catalyst for transesterification, heterogeneous catalysts have been examined from the viewpoint of separating a catalyst-component after reaction. For example, there are known a method for utilizing an inorganic solid acid such as silica-alumina and zeolite (JP-A 61-200943), a catalyst containing aluminum oxide and/or iron oxide (JP-A 61-236749), a method for using a silicate or the like of the group IV elements (EP0623581A2), a method for utilizing an organic solid acid such as an ion-exchange resin (WO98/25876) and a method for using hydrous zirconium oxide (JP-B 4-28250).
However, the heterogeneous catalysts described above could not satisfy all of activity, selectivity and durability in transesterification. Specifically, the inorganic solid acid has strong acid-strength, and for example, a zeolite such as mordenite gives rise to significant formation of an undesirable by-product such as an ether upon transesterification. The activity of the silicates of the group IV elements is relatively low, and thus there is a restriction that high temperature conditions must be adopted. Further, elution of silicon occurs during the reaction to permit a reduction in the activity during a long operation. The ion-exchange resin as an organic solid acid is low not only in activity but also in thermostability, and thus there is a limit upon its usable temperature range. Nevertheless, the patent using the ion-exchange resin describes the condition that the reaction is conducted in a gaseous phase, adjusting temperature and pressure. Accordingly, a catalyst using the ion-exchange resin is applicable to only low-boiling and low-molecular reactants and cannot be applied to high-boiling reactants. On one hand, the catalyst using the ion-exchange resin has the problem of swelling upon contact with lower alcohol. The utilization of the hydrous zirconium oxide is disclosed as a means of improving selectivity, but its industrial application is not achieved due to low activity thereof.
Accordingly, the object of the present invention is to provide a heterogeneous catalyst which is highly active, excellent in selectivity and free from elution of its catalytically active components and which has long lifetime.
The present invention relates to a catalyst for transesterification comprising a phosphate of at least one metal selected from the group consisting of aluminum, gallium and iron. Aluminum is particularly preferable.
Preferably, the molar ratio of the metal:phosphoric acid is from 1:3 to 1:0.01.
The catalyst may comprise a boric acid-group or an alkaline earth metal.
Also, the present invention relates to a method for producing a catalyst for transesterification, which comprises bringing a solution containing a phosphate ion into contact with at least one of an oxide, a hydroxide and a nitrate of at least one metal selected from the group consisting of aluminum, gallium and iron.
The above-mentioned method comprises preferably calcining, further.
In the above-mentioned method, the molar ratio of the metal ion to phosphate ion is preferably from 1:2 to 1:0.01.
The present invention relates to a method for producing an ester compound, which comprises transesterifying an alcohol, a carboxylic acid or an ester compound with a starting ester in the presence of a catalyst comprising a phosphate of at least one metal selected from the group consisting of aluminum, gallium and iron.
The present invention preferably provides a method for producing an ester compound, which comprises transesterifying an alcohol, a carboxylic acid or an ester compound with a starting ester in the presence of the catalyst as defined above.
In the above-mentioned method for producing an ester compound, the catalyst preferably comprises a boric acid-group or an alkaline earth metal.
In the above-mentioned method for producing an ester compound, the catalyst is preferably that obtained by the process as defined above.
The said catalyst can also be prepared from a metal phosphate and an alkaline earth metal, boric acid or a borate.
Further, the present invention relates to use of a phosphate of at least one metal selected from the group consisting of aluminum, gallium and iron as catalyst for transesterification.
In general, a phosphate may be in many forms such as orthophosphate, polyphosphate, metaphosphate and pyrophosphate. Then, as the catalyst of the present invention, orthophosphate is preferably used. However, a phosphate in other form may also be included without hindrance. The metal forming the phosphate of the present invention is one or more members selected from the group consisting of aluminum, gallium and iron. Among them, aluminum is particularly preferable. Further, two or more metal phosphates may be used as complex, and such metals are not limited the metals mentioned above.
As to the composition of the catalyst, the molar ratio of the metal ion to the phosphate ion is preferably 1:3 to 1:0.01, more preferably 1:3 to 1:0.1 and especially preferably 1:1.2 to 1:0.2, from the viewpoint of catalytic activity.
The phosphates used in the present invention may be commercially available or may be obtained by preparation. The form of the phosphate may be any of amorphous and crystalline one. For example, one of the amorphous phosphates can be obtained by allowing an alkaline substance to act on a mixture of a metal nitrate-solution and phosphoric acid, thereby obtaining a precipitate and following up with treatments such as filtration, washing with water, drying and calcination. The precipitate can be used as the catalyst in a dried form without subjection to calcination. In this case, formation of ethers is suppressed.
From the viewpoint of activity, it is preferable to calcine the catalyst for 0.1 hour or more at 150xc2x0 C. or higher.
As to the composition of the catalyst obtained by precipitation, the molar ratio of the metal ion to the phosphate ion is preferably 1:3 to 1:0.1, more preferably 1:1.2 to 1:0.2, from the viewpoint of catalytic activity.
In preparing a metal phosphate or a complexed metal phosphate, a carrier (which may be a supporting member) having its large surface-area may be coexist to prepare an immobilized (or supported) metal phosphate or an immobilized (or supported) complexed metal phosphate on the carrier. The carrier may be used as one which is generally used for carrier such as silica, alumina, silica-alumina, titania, zirconia and an activated carbon. If the carrier is used in excess, the content of the phosphate as its active component is lowered to make the activity low. Accordingly, the ratio of the carrier to the catalyst is preferably not more than 90% by weight.
The method for making a phosphate ion adhere to a metal oxide etc. by bringing a solution of the phosphate ion into contact with the metal oxide etc. includes the method (a) in which the metal oxide etc. are dispersed in the solution of the phosphate ion and a solvent thereof is evaporated from the dispersion to allow the phosphate ion to adhere to the metal oxide etc. as they are (impregnation), the method (b) in which the solution of the phosphate ion is passed through metal oxide etc. followed by drying, and the method (c) in which metal oxide etc. are impregnated with the solution of the phosphate ion in an amount corresponding to the pore capacity of the metal oxide etc. followed by drying (Incipient Wetting Method). Among them, the method (a) is generally used.
The phosphate ion may be added in such an amount that a ratio of phosphorus atom to metal atom is 0.01 to 2.0, in the catalyst obtained by calcination. It is more preferable that the ratio is 0.05 to 1.0.
Calcination of the catalyst is preferably conducted at a temperature in the range of 150 to 1000xc2x0 C. and it is more preferably done in the range of 200 to 800xc2x0 C. The effect can be sufficiently obtained by means of calcination in a short time under this temperature condition, but the said temperature is preferably maintained, it is more preferably done for 0.5 hour or more, and it is further preferably done for 1.0 hour or more.
Examples of the crystalline phosphate include a crystalline ammonium phosphate such as VPI-5 and AlPO-5. VPI-5 is a crystalline aluminum phosphate having its pore diameter of 1.2 to 1.3 nm and can be obtained by a productive process described in the literature (ZEOLITES, vol.12, p.2, 1992). AlPO-5 is also an aluminum phosphate having crystallinity and can be obtained by a synthetic process described in the literature (Shokubai (Catalyst), vol. 27, p. 251, 1985).
Further, a complexed phosphate catalyst can be prepared in coexistence of two or more metals during preparation.
For example, the effect of improving selectivity such as suppressing an ether can be obtained by adding an alkaline earth metal. The alkaline earth metal is preferably magnesium, calcium, strontium and barium. Among them, magnesium and calcium are particularly preferable.
From the viewpoint of further improvements in catalytic activity, the catalyst can be prepared by allowing boric acid and/or a borate to be coexistent besides phosphoric acid during preparation. Specifically, boric acid-groups can be contained such that the content of boric acid-group, as defined in Equation (1), is in the range of 0.1 to 60% by molecule.                                                                         content                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                boric                                                                                        acid                ⁢                                  -                                ⁢                group                                                    =                                            Number of boron atoms                                                      (                                                                                                    number                        ⁢                                                  xe2x80x83                                                ⁢                        of                                                                                                                                                boron                        ⁢                                                  xe2x80x83                                                ⁢                        atoms                                                                                            )                            +                              (                                                                                                    number                        ⁢                                                  xe2x80x83                                                ⁢                        of                                                                                                                                                phosphorus                        ⁢                                                  xe2x80x83                                                ⁢                        atoms                                                                                            )                                              xc3x97          100                                    (        1        )            
In case the catalyst is powder, the catalyst can be used by dispersing the catalyst, as it is, in a raw material. On the other hand, the catalyst can be used after molding a form of the catalyst. A proper binder may be used at molding. When the catalyst being powder is used, the catalyst is separated by filtration from the reaction-product after the reaction. When the catalyst being a molded catalyst is used, it is possible that a continuous reaction is conducted by charging the catalyst into a reaction-column.
The transesterification is conducted by mixing a starting ester with a starting alcohol, a starting ester with a starting carboxylic acid, or a starting ester with another starting ester and then brining the resultant mixture into contact with the catalyst under a reaction-condition. The starting ester, the starting alcohol and the starting carboxylic acid for use are not particularly limited.
For example, the starting ester may be an ester or a partial ester of a C1 to C22 linear or branched aliphatic carboxylic acid or aromatic carboxylic acid or a mixture thereof and a C1 to C22 linear or branched monohydric alcohol or polyhydric alcohol. More specifically, it may be, for example, an ester of a carboxylic acid such as acetic acid, butyric acid, caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, oleic acid and stearic acid or a dicarboxylic acid or a mixture thereof and a monohydric aliphatic alcohol such as methanol, ethanol, propanol, butanol, octanol and stearyl alcohol, a monohydric aromatic alcohol such as benzyl alcohol, or a polyhydric alcohol such as ethylene glycol, propylene glycol, glycerol, pentaerythritol and sorbitol. Examples of such an ester include a natural vegetable oil such as monoglyceride, diglyceride, triglyceride, coconut oil, palm oil and palm-kernel oil and an animal oil such as a beef-tallow and a pork-lard.
Then, as the starting alcohol, a C1 to C22 linear or branched monohydric alcohol or polyhydric alcohol is used. More specifically, it maybe instanced as a monohydric aliphatic alcohol such as methanol, ethanol, propanol, butanol, octanol and stearyl alcohol, a monohydric aromatic alcohol such as benzyl alcohol and a polyhydric alcohol such as ethylene glycol, propylene glycol, glycerol, pentaerythritol and sorbitol.
Then, as the starting carboxylic acid, a C1 to C22 linear or branched aliphatic carboxylic acid or aromatic carboxylic acid is used. More specifically, it maybe, for example, instanced as acetic acid, butyric acid, caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, oleic acid and stearic acid.
The charging ratio of the starting ester to the starting alcohol, the starting carboxylic acid or the other starting ester (hereinafter referred to as starting alcohol etc.) can be varied depending on the required purity of the resultant ester product. That is, in case it is allowable that a certain amount of the starting ester is present in the product to be impure, the fed amount of the starting alcohol etc. may be the stoichiometric amount or less. Namely, the consumed amount of the starting alcohol etc. can be reduced. On the other hand, in case a higher purity of the ester product is desired, it is possible that an excess of the starting alcohol etc. is fed so as to shift equilibrium to ester. Accordingly, a higher yield can be obtained.
As the method of transesterification, a conventional method can be used, as it is. For example, the starting ester and the starting alcohol can be continuously fed to a reaction-column charged with the catalyst or can be reacted in batch-wise in a reaction chamber. Further, in case a separated liquid such as glycerol is generated as the reaction proceeds, it can also be separated and removed continuously or intermittently. In addition, the reaction can be carried out under the condition of ordinary pressure or pressurization. The condition of pressurization can accelerate alcohol liquefying so that it is kinetically advantageous.
The catalyst of the present invention is excellent in thermostability and stable even at 400xc2x0 C. or more. Accordingly, there is no particular limit to the condition of the used reaction-temperature. Further, the catalyst is insoluble in the starting ester and the starting alcohol etc. and thus elution of its active component does not take place. Therefore, the reaction can be carried out in both of gas phase system and liquid phase system.
According to the present invention, a heterogeneous catalyst can be obtained, which is highly active, excellent in selectivity and free from elution of its catalyst-component and which has long lifetime. By using the present catalyst, a high quality ester can be continuously produced by transesterification for a long period, easily separating the product from the catalyst.